Abstract
The present work presents an improved method to align the measurement scale mark in an immersion hydrometer calibration system of CENAM, the National Metrology Institute (NMI) of Mexico, The proposed method uses a vision system to align the scale mark of the hydrometer to the surface of the liquid where it is immersed by implementing image processing algorithms. This approach reduces the variability in the apparent mass determination during the hydrostatic weighing in the calibration process, therefore decreasing the relative uncertainty of calibration.
Highlights
Hydrometers are widely used instruments in industry to measure the density of liquids
The system that has been developed in CENAM for hydrometers calibration, includes a thermostatic bath with an external temperature controller, a glass vessel to contain the reference liquid, a platinum resistance thermometer to measure the temperature of the liquid, an hygro-thermometer, a barometer to obtain the density of air, and a set of mass standards and a weighing instrument capable to perform hydrostatic weightings, as can be seen on Figure 1
Once the mass in the air is measured and the points of the scale to be calibrated are selected, the hydrometer is immersed in the reference liquid
Summary
Hydrometers are widely used instruments in industry to measure the density of liquids. One of the difficulties in calibration by Cuckow’s method, when the hydrometer is immersed in the liquid of known density, is how to align the desired scale mark of the hydrometer with the liquid surface because of the meniscus formed around the hydrometer stem caused by the surface tension of the liquid. This alignment depends on the skill of the metrologist, giving rise to human errors that have a direct impact in the best estimate and in the uncertainty of the calibration result. Key and supplementary international comparisons have been carried out to validate the methodology proposed in this work
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